Stator of a moineau-pump

ABSTRACT

A Moineau stator includes a tube ( 10 ) having lobes ( 3 ) arranged in a configuration which is adapted to interact with a rotor and formed through a hydroforming process.

FIELD OF THE INVENTION

The present invention relates to a method of forming a Moineau Statorand a Moineau Stator formed in accordance with the teachings of themethod.

BACKGROUND OF THE INVENTION

PC pumps and mud motors (“Moineau pumps”) of conventional design have amoulded elastomeric insert bonded firmly to the inside of a cylindricalcase, usually made of steel. This comprises the stator of the pump ormotor unit. The inside shape of the elastomer is formed with a cavitythat has a helical characteristic that mates with a helically-shapedstator. Interference between the two components creates seal lines thatcontain cavities of fluid which progress in the axial direction when therotor is rotated relative to the stator. If rotational power is appliedto the rotor, the assembly functions as a pump against differentialpressure. If differential pressure is applied across the assembly,rotary power is extracted from the rotor and the assembly functions as amotor.

When formed inside of a cylindrical case out of elastomer, the shape ofthe stator cavity requires the elastomer thickness to vary around thecircumference. The locations where the thickness is greatest aresubjected to the largest distortional elastomer stresses duringoperation.

Cyclic stress developed in the elastomer by the seal location movingback and forth, or around the stator cavity generates heat in the coreof the elastomer, which must be removed by conduction through theelastomer, either to the outer stator casing or to the inner surface ofthe elastomer where it is convected to the transported fluid. Inconventional designs, the largest heat-generation rate occurs where theability to remove the heat is lowest. If it over-heats, the elastomercan fail and the function of the pump/motor is compromised. This hasbeen a significant limitation in the performance and design ofprogressing cavity pumps and motors, and has led to the development of“uniform-thickness” elastomer designs, where the internal casing profileis provided to closely match the required stator cavity profile, and arelatively thin layer of elastomer is moulded to this surface to providethe final stator cavity geometry.

This approach has several advantages, including reduced heat generationand swelling characteristics. The primary disadvantage is the cost ofproviding the relatively complicated internal profile from thehigh-strength material of the casing. Several approaches have beendeveloped, including cold-rolling techniques, machining of the internalprofile, and the use of extrusion techniques to produce the requiredgeometry. These approaches are expensive, particularly in the lengthsrequired for PC pump/motor applications. Some of these techniques aredescribed in Canadian Patents 2,315,043 (Krueger et al), 2,333,948(Underwood et al) and U.S. Pat. No. 6,427,787.

Furthermore, while these patents identify certain advantages to begained from thin walled stators, the methods of manufacture described,are not amenable to close tolerance control for such stators.

SUMMARY OF THE INVENTION

What is required is an alternative method of forming a profiled Moineaustator, where such method supports the forming of a thin walled profiledMoineau stator.

According to the present invention there is provided a method of forminga Moineau stator with a prescribed interior profile. A first stepinvolves placing a ductile metal tube into a hydroforming fixture. Asecond step involves forming the tube to have lobes through ahydroforming process. The lobes are arranged in a configuration which isadapted to interact with a rotor.

In order to ensure efficient fluid movement, it is preferred that afurther step be taken of coating the interior of the tube with anelastomer layer adapted to form a fluid seal with a rotor. As willhereinafter be described, hydroforming is a very cost effectivealternative to previously known methods of forming profiled Moineaustator cases suitable for lining with a uniform thickness elastomericlayer. Although using this method, the elastomer coating on the interiorof the tube need not be uniform.

According to another aspect of the present invention there is provided aMoineau stator which includes a tube having lobes arranged in aconfiguration which is adapted to interact with a rotor and formedthrough a hydroforming process. It is preferred that the tube has anelastomer coated interior adapted to form a liquid seal with a rotor.This elastomer coating may be of uniform thickness or may intentionallybe made unequal to create a preferential distribution of elastomercoating at intervals along the axial length of the tube.

The beneficial results obtained through the use of the Moineau stator,as described above, may be further distinguished as this method can beused with both thick walled and thin walled embodiments. The greaterrigidity and strength of thick walled embodiments supports containmentof greater pressure differential than thin walled embodiments, whilethin walled embodiments enjoy the benefit of reacting a significantportion of the seal interference through non-heat generating deformationof the tube wall rather than mostly as heat generating elastomerdeformation.

It is therefore preferred that thin walled embodiments be surrounded bya coaxially positioned support housing capable of reacting the majorityof the total pump or motor pressure differential. This support housingcan either be cylindrical or may have lobes, at least on its interiorsurface, where said interior lobes are arranged as if comprising anadditional external stator in relation to the lobed stator exterior asif acting as a rotor. Means to transfer radial load from the exterior ofthe thin walled stator to the interior of the support housing isprovided largely by material placed in the annular space between thestator and support housing arranged to limit the pressure differentialacross the thin walled stator to prevent its excess expansion orcollapse. The material placed in the annular space is preferably a fluidwith means to control its pressure. The annular space is more preferablyarranged to allow for a variation of the annular fluid pressure alongthe stator length to generally equalize the pressure between the annulusand stator interior. Variation of the annular fluid pressure issupported by providing a plurality of generally axially distributeddiscrete cavities, sealing segregated from each other.

When the support housing has internal lobes arranged in relation to thethin walled stator as described above, it will be appreciated that aplurality of generally axially distributed cavities is formed. In suchcase it is preferred that the tube have an exterior surface coated withelastomer to more readily sealingly engage the interior surface of thelobed support housing and thus provide a more positive fluid sealbetween adjacent cavities.

When the support housing is provided as a cylinder, one or more axiallydistributed bulkheads are placed in the annulus between the tube and thesupport housing. Said bulkheads and arranged to attach to at least oneof and sealingly engage both the tube and support housing thus creatingaxially distributed discrete cavities.

There are various means which can be used to equalize pressure betweenthe cavities thus formed and the stator interior. There will hereinafterbe illustrated and described a method which involves providing fluidpassages which allow fluids from the interior of the tube to communicatewith the axial cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings, the drawings are for the purpose of illustration only and arenot intended to in any way limit the scope of the invention to theparticular embodiment or embodiments shown, wherein:

FIG. 1 is a perspective view of the uniform thickness Moineau statorfabricated in accordance with the teachings of the present invention.

FIG. 2 is a perspective cut-away view of a stator hydroforming fixtureconstructed in accordance with the teachings of the present invention.

FIG. 3 is a side elevation view, in section, of the stator hydroformingfixture illustrated in FIG. 2 with tube inserted ready for forming.

FIG. 4 is a side elevation view, in section, of the stator hydroformingfixture illustrated in FIG. 2 with tube after the forming process hasbeen concluded.

FIG. 5 is a cross-sectional view of a uniform thickness Moineau statorwith thick walls fabricated in accordance with the teachings of thepresent invention.

FIG. 6 is a cross-sectional view of a uniform thickness Moineau statorwith thin walls fabricated in accordance with the teachings of thepresent invention.

FIG. 7 is a cross-sectional view of a variable elastomer thicknessMoineau stator with thick walls fabricated in accordance with theteachings of the present invention.

FIG. 8 is a cross-sectional view of the uniform thickness Moineau statorwith thin walls illustrated in FIG. 6, with a cylindrical supporthousing.

FIG. 9 is a side elevation view, in section, of the uniform thicknessMoineau stator with thin walls illustrated in FIG. 6, with a cylindricalsupport housing and discrete pressurized axial cavities.

FIG. 10 is a cross-section view of the uniform thickness Moineau statorwith thin walls illustrated in FIG. 6, disposed within a multi-lobedsupport housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment, a uniform elastomer thickness Moineau statorgenerally identified by reference numeral 10, will now be described withreference to FIGS. 1 through 10.

Referring now to FIG. 1, a stator 10 is shown comprised of a stator body1 formed from a metal tube having a sidewall 2 into which a plurality ofhelically symmetric lobes 3 are placed, illustrated here as it wouldappear configured in a four lobe Moineau stator. An elastomeric liner 4is disposed on the inside surface 5 of the stator body 1. The lobes areplaced by a specialized stator hydroforming process.

Hydroforming is a manufacturing method that generally uses fluidpressure to deform a ductile metal shell against a mold. To form shapessuch as required for stators 10, the mold can take a number of helicaland solid forms, configured so that the post-hydroformed internalprofile of the stator housing obtains the general form of the lobedprofile of the inner surface of the elastomer. If necessary, the partmay be heat treated after forming to relieve residual stresses, providedthis process does not change the dimensional tolerances so the part isunusable. The desired stator profile may be achieved by hydroformingusing either internal or external pressure to deform the tube.

Referring now to FIG. 2, in its preferred embodiment a hydroformingfixture 100 is provided to implement said preferred stator hydroformingprocess by application of internal pressure. The fixture is essentiallya coaxial assembly of close fitting largely cylindrical components.Beginning with the innermost and progressing outward, these componentsare: a mandrel 101, stator body 1 as a work piece (provided as a metaltubular ‘blank’), a mold assembly 103 comprised of elements as necessaryto allow removal after forming, an externally tapered collet 104comprised of an assembly of jaws 105 and a confining vessel or bell 106a thick-walled pressure vessel capable of containing the formingpressure and internally tapered to mate with the collet. Additionally, ameans to apply axial displacement between the collet 104 and bell 106 isprovided, such as a double acting hydraulic actuator (not shown). Aswill be apparent to one skilled in the art said axial displacement isconverted to radial displacement by the collet jaws 105 moving incontact with the bell 106 facilitating installation and removal of theclose fitting parts.

Referring now to FIG. 3, the mandrel 101 is provided with internal seals110 engaging the inside bore 2 of the work piece blank 1 and a fluidentry port 111 in communication with the mandrel exterior 102 betweenthe seals 110. Fluid applied through this port is thus contained by themandrel 101, it being in sealing engagement with the work piece 1,allowing application of pressure to the internal surface of theworkpiece 1 by suitable means such as may be provided by a high pressureair over hydraulic pump.

Referring now to FIG. 4, application of sufficient pressure through port111 causes the work piece 1 to expand and plastically deform unlessconstrained by contact with the internal surface of the contained mold,thus inflating the sidewall of the work piece 1 into the mold cavities107 to form lobes 3 in the stator body 1. The portion of the pressureforce reacted by the mold 103 is in turn reacted through the collet 104into the bell 106. Due to the tapered interface between the collet 104and bell 106, the collet 104 may tend to slip in the bell 106 whileunder pressure load allowing unwanted expansion of the work piece 1.This movement may be readily prevented by application of axial load orother suitable means of restraint between the collet jaws 105 and bell106. Upon removal of the forming pressure, the mandrel 101 is readilyremoved, however a residual radial stress or interference may existbetween the work piece 1 and mold assembly 103 tending to resist removalof the work piece 1 and mold assembly 103 from the collet 104. Thisradial stress is relieved by appropriate displacement of the colletrelative to the bell enabling removal of the work piece 1 together withthe components of the mold assembly 103, since the formed lobes 3 areinterlocking with the mold cavities 107 after forming. Once removed fromthe forming fixture 100 the mold assembly 103 may be removed from theformed stator body 1.

The hydroforming fixture 100 is preferably long enough to ensure thatthe profiled stator 10 can be formed as a single piece. Alternately, thestator may be formed in short lengths and assembled into a completeunit, with the length depending on the required pressure capacity of thepump or motor. If necessary, the forming process on any one piece couldbe performed in more than one step (i.e., multiple hydroforming stepsusing different die sets) to ensure that a preferential distribution ofplastic strain is achieved in the housing.

With reference now to FIG. 1, it will be appreciated that furtherfinishing of the ends 6 & 7 of the stator body 1, hydroformed accordingto the teachings of the present invention, will generally be required toenable attachment of the stator 10 to other elements of the pump ormotor, or drill string or tubing string supporting said pump or motor.The end geometries must accommodate insertion of the rump or motor rotorand any other components that must pass through the stator. The correctgeometry may either be incorporated into the hydroforming design orcould be fitted after forming is complete. In one embodiment, duringhydroforming, the end sections of the stator tube 6 & 7 are held at theinitial (unformed) diameter to enable sealing with the mandrel 101.After the formed tube is removed from the fixture, these ends are cutoff. Required connections to other components of the string can beachieved through welding or other means.

The inner elastomer layer 4 may be applied to the stator body 1 byvarious means known to the industry but is preferably placed byinjection moulding. Referring again to FIG. 4, the hydroforming fixture100 supports this operation which may require internal pressure greaterthan can be born by the unsupported stator body 1. To complete thistask, a mandrel defining the inner profile of the elastomer iscentralized inside the formed tube, and the elastomer injected accordingto standard injection moulding practice.

According to the needs of various applications, the hydroformed statorbody 1 may be manufactured in both thin-wall and thick-wallconfigurations as understood in the art. Referring now to FIG. 5, inthick-wall implementations the thickness of the hydroformed stator body1 sidewall 2 is selected so that it is substantially rigid underapplication of rotor contact loads and preferably has sufficientstrength to react the pressure differential that may arise in use of thestator 10 in a pump or motor. As shown in the cross section view of FIG.5, the external profile of the hydroformed thick wall stator body 1generally has the same character as its internal profile. This istypically the most space-efficient design, and the external profileoffers several advantages in use, including reduced flow loss throughthe external annulus formed when the stator is placed within a well, andadded flexibility for installation options. In this case, the thicknessof the stator body 1 must be adequate to support the torsional and axialloads generated during operation in addition to the associated internalfluid pressure.

Referring now to FIG. 6, a hydroformed stator 10 is shown in crosssection as it would appear in its thin wall configuration. (Thick andthin wall representations between FIGS. 5 and 6 are only intended toillustrate relative proportions of the stator body 1.) In thisconfiguration, the thickness of the stator body 1 sidewall 2 is selectedso that it will deflect under application of the rotor interferenceload, thus contributing a portion of the compliance required toaccommodate the interference effecting the seal contact stress. This isadvantageous as a means to reduce the demands placed on the elastomerlayer 4, however it simultaneously reduces the pressure capacity of thestator body 1.

In addition to the benefits obtained from an elastomer of uniform wallthickness, additional benefits may be obtained where the elastomerthickness is selected to vary such that the performance characteristicsof the motor or pump (fluid seal quality and consistency, heatgeneration and dissipation in the elastomer, elastomer/housing bondperformance) are optimized. Referring now to FIG. 7, the elastomer 4 isshown to have a variable circumferential thickness, with the thicknessbeing larger at the major seal locations 8 and smaller at the minor seallocations 9. In an application that is particularly sensitive to heatgeneration, the elastomer thickness at the major seal could be selectedto be greater than that at the minor seal. This would make the majorseal more compliant than if the elastomer thicknesses were consistentand would reduce the sensitivity of the heat generation rate to rotorand/or stator dimensional tolerance variations. As will be apparent toone skilled in the art, other optimizations pertaining to performancecould be achieved by varying the circumferential and/or axialdistribution of elastomer thickness. The hydroforming fixture 100readily supports such control of elastomer thickness distribution, bymodifying the geometry of mold assembly 103 in coordination withselection of the internal pressure.

In applications where such reduced pressure capacity is insufficient,the stator 10 is preferably supported by a secondary containment vessel.In one embodiment, the secondary containment vessel is provided as acylinder. Referring now to FIG. 8, in this embodiment, a supported thinwall stator assembly 200 is shown in cross section where, the thinwalled stator body 1 is coaxially placed inside a cylindrical supporthousing 201 forming an internal annulus 202. With this configuration,the stator body 1 is readily supported as required by a filler toprevent its excess expansion or collapse by providing means to transferradial load across the annulus 202. Such filler may be provided byplacing a compliant but relatively incompressible solid such as anelastomer in the annulus 202. Alternately radial load transfer isreadily provided by fluid pressure in the annulus 202 where, in a mannerknow to the art, end closures are provided to sealingly attach the endsof stator body 1 to the cylindrical support housing 201 and the annulus202 allowed to communicate with various of the fluid pressure points inthe pump or motor application.

However, the fluid pressure is more preferably arranged to vary alongthe length of the stator 10 to generally equalize the pressure betweenthe annulus and stator interior. It will be appreciated that control ofpressure in these annulus cavities provides a means to reduce thepressure drop across the stator 10 and thus prevent overload of thestator body 1.

One novel means to provide such graduated pressure support is describednow with reference to FIG. 9 showing an interval of a supported thinwalled stator assembly 200. Variation of the annular fluid pressure issupported by providing a plurality of generally axially distributeddiscrete cavities 203, sealing segregated from each other by bulkheads204. The position of bulkheads 204 is maintained by spacers 205contained within the support housing 201 and associated end closures.This configuration also provides a simple means of achieving accurateseal element spacing. Pressure equalization is provided by ports 206.

Referring now to FIG. 10, in an alternate even more novel embodiment, asupported thin wall stator assembly 300 is shown in cross section wheregraduated pressure support is enabled by providing the support with alobed support housing 301 configured in a hypocycloid geometrycompatible with the stator 10 so that the stator 10 can be easilyinserted into the lobed support housing. In this case, the lobed supporthousing 301 has one more lobe than the primary housing and a pitchdefined by the ratio of secondary to primary hypocycloid lobes. Sealsbetween cavities are generated either through metal-to-metal seals or(more likely) through contact with an intermediate elastomer layer 302applied to the outside of the stator 10 or inside of the lobed supporthousing 301. The cavities 303 are ported to the transported fluid toprovide pressure equalization as required to prevent excess deformationof the stator 10. The cavities that terminate at either end of the motorsection may be sealed to reduce risk of fluid migration along thecavities.

By providing a thin-walled stator 10 with a secondary housing, thestator housing geometry will be less expensive to fabricate than asingle thick-walled primary housing. Using a formed secondary housingcould simplify the task of creating an axial pressure distribution inthe stator housing annulus provided the overall size of the motor is notprohibitive. Both of these approaches would provide additionalcompliance at the rotor/stator seal lines to accommodate tolerances,swelling and thermal expansion. This is a significant advantage overconventional uniform-wall designs, where the stiffness of the thinelastomer layer has low tolerance for such variations. Indeed, carefuldesign of the thin-wall stator could reduce the required elastomerthickness or eliminate the requirement for an elastomer completely inmany applications.

Another embodiment of this essential theme is a thin-walled design witha supporting structure provided by a high-strength composite wrap thatcan carry the full differential pressure between the transported fluidand the surrounding fluid. The thickness of this wrap might vary overthe pump/motor length consistent with the variation in differentialpressure over the length.

1. A Moineau stator, comprising: a tube (10) having lobes (3) arrangedin a configuration which is adapted to interact with a rotor, the tube(10) is thin-walled with walls (2) that are sufficiently thin as to besubjected to elastic deformation in response to interfacial seal forcesimposed by interference with the rotor and is surrounded by a supportingstructure (201) in the form of a support housing having walls able toresist pressure, torque, and axial loads experienced in its intendedoperating environment, discrete pressurized axial cavities (203) arepositioned in an annulus (202) between the tube (10) and the supporthousing (201) and fluid passages (206) are provided to equalize pressurein the axial cavities (203) with pressure within the interior (5) of thetube (10) by allowing fluids from the interior (5) of the tube (10) tocommunicate with the axial cavities (203).
 2. The Moineau stator asdefined in claim 1, wherein the support housing (301) has lobes arrangedin a configuration adapted to interact with the lobes on the tube (10)to form said discrete pressurized axial cavities, thereby balancingpressure acting on the interior surface (5) of the tube (10) with asubstantially equal pressure acting on the exterior surface (6) of thetube (10) such that the deformation of the tube (10) in response topressure variations is limited.
 3. The Moineau stator as defined inclaim 2, wherein one of an exterior surface (6) of the tube (10) or aninterior surface of the support housing (301) is coated with elastomer(302).